Electric fluid heater and resistance heating element therefor

ABSTRACT

An electric fluid heating element comprises a column-shaped honeycomb structural body of an electrically conductive cermaic material having a positive temperature coefficient of resistance. The body has a multiplicity of substantially uniform parallel channels extending therethrough with each of the channels being bounded by a partition wall which is substantially uniform in thickness. The body has a surface to volume ratio in the range of 10 to 60 cm2/cm3 and is selfcontrolling without the necessity of a safety device such as a fuse or thermostat. Ohmic electrodes are mounted on oppsoite surfaces of the body to supply heating current thereto. The heating element can be used as the heating means in an air heater, humidifier or liquid heater.

4 United States Patent Wada et a1. Dec. 16, 1975 1 ELECTRIC FLUID HEATER AND 3,265,865 8/1966 Hager 219/300 x 2 21828 .1121: 212/221 u1 mg e a THEREFOR 3,691,346 9/1972 Dyre et al. 219/374 [75] Inventors: Shigetaka Wada, Kuwana; Noboru Yamamoto Nagoya, both of Japan FOREIGN PATENTS OR APPLICATIONS 512,667 10/1920 France .1 219/374 1 Asslgneel NGK Insulators, Nagoya, 932,558 7/1963 United Kingdom 219/300 Japan 499,074 l/l939 United Kingdom 219/382 [22] Filed: Mar. 4, 1974 Primary Examiner-A. Bartls [21] Appl. No.: 447,513

[57] ABSTRACT [30] Foreign Apphcatlon Pnonty Data An electric fluid heating element comprises a column- Mar. 9, Japan haped honeycomb tructural of an electrically conductive cermaic material having a positive temper- U-S- Clature oefficient of resistance The has a multi- 3 1 5 3355/22 252/520 plicity of substantially uniform parallel channels ex- [51] Cum H053 3/14; HOlC 7/02; F24H 1/10; tending therethrough with each of the channels being F24H 3/04 bounded by a partition wall which is substantially uni- [58] Field of Search 219/381, 382, 374376, f in thickness The body has a Surface to volume 23, ratio in the range of 10 to 60 cm /cm and is selfcon- 362, 58; 252/518 520 trolling without the necessity of a safety device such as a fuse or thermostat. Ohmic electrodes are mounted [56] References Cited on oppsoite surfaces of the body to supply heating UNITED STATES PATENTS current thereto. The heating element can be used as 927,173 7/1909 Schluter 219/381 the heating means in an air heater, humidifier of liquid 1,334,809 3/1920 Simon et a1. 219/381 UX heaten 3,163,841 12/1964 Willett 219/374 UX 8 Cl 8 D F. 3,244,860 4/1966 Lindley 219/382 x rawmg gums OHMIC ELECTRODE PTC CERAMIC MATERIAL OHMIC ELECTRODE sheet 1 of5 3,927,300

U.S. Patent Dec. 16, 1975 OHMIC ELECTRODE PTC CERAMIC MATERIAL ommc ELECTRODE PTC CERAMIC MATERIAL OHMIC ELECTRODE U.S. Patent Dec. 16, 1975 Sheet20f5 3,927,300

m? SURFACE TO VOLUME RATIO ABOUT 24 CM2/CM3 E 200 \SURFACE T0 VOLUME RATIO Q ABOUT l6 cm /cm 0./ 0:2 0:5 014 0'5 O(m /min) Sheet 3 of 5 3,927,300

US. Patent Dec. 16, 1975 US. Patent Dec. 16,1975 Sheet40f5 3,927,300

US. Patent Dec. 16,1975 Sheet50f5 3,927,300

ELECTRIC FLUID HEATER AND RESISTANCE HEATING ELEMENT THEREFOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to heating elements, and more particularly to a heating element comprising a ceramic article having a positive temperature coefficient of electric resistance (hereinafter referred to as PTC ceramic article). The element is composed of a honeycomb structural body and includes a pair of ohmic electrodes. The element is adapted to generate a large amount of heat radiation with a small volume without any risk of overheating and breaking. The invention also includes a heater comprising this heating element which is particularly useful as an air heater, dryers inclusive of a hair dryer, air towel, liquid heater, humidifier, volatilizer and the like.

In the in the remainder of the specification, the term honeycomb structural body shall be understood to mean a structural body having a multiplicity of channels extending therethrough and generally parallel to each other and having high surface-to-volume ratios.

2. Description of the Prior Art Heretofore, it has been the common practice to use metals and ceramics having a negative or a positive temperature coefficient of electric resistance.

Metal has a small specific resistance and hence is commonly used in linear form in the heating element. If such heating element is used, for example, in a hair dryer and a fan is used to feed air therethrough, and suction opening is clogged, the metal constituting the heating element becomes overheated, causing a fire. Eventually there is a risk of the metal being oxidized and broken.

The ceramic article having a negative temperature coefficient of electric resistance, is commonly formed into a rod having a pair of ohmic electrodes mounted thereon and used as a heating element. A ceramic article for example, silicon carbide, in order to generate a given amount of heat radiation without overheating the heating element per se, a number of these heating elements must be used simultaneously. Alternatively temperature control means must be used, and as a result, the assembly becomes complex in construction and/or there is a risk of the heating element being broken. Thus, such a heating element is not suitable for use with domestic heaters.

In addition, a PTC ceramic article made of semiconductive barium titanate has commonly been formed into a disc-shaped pellet on which are mounted a pair of ohmic electrodes for use as a heating element. Such kind of heating element, however, can only generate the amount of heat radiation corresponding to a few watts from one heating element. As a result, in order to cause it to generate the amount of heat radiation corresponding to several watts, a large heat radiating plate must be added thereto. Thus, this kind of heating element is subjected to restrictions in its construction. Moreover, if it is desired to obtain a large amount of heat radiation with the aid of a high electric power of, for example, more than 1 KW, a number of heating elements each including the above described heat radiating plate are required. The use of the measures described is extremely uneconomical and results in considerable constructional disadvantage as that it could not be applied to domestic heat radiators.

In addition, a heating element comprising a cylindrical PTC ceramic article which is provided at its inner and outer surfaces with ohmic electrodes has been known. If this heating element has its surface area of about 1,000 cm in order to obtain a sufficiently large amount of heat radiation, for a the diameter of 5 cm it is necessary to make the length longer than 30 cm. As a result, the volume of the heating element becomes too large as compared with the volume of the customary type of heating element. Thus, the mechanical strength of the heating element becomes lowered and this type of heating element has not achieved any importance in actual practice.

SUMMARY OF THE INVENTION The invention is based upon recognition of the fact that a PTC ceramic article composed of a honeycomb structural body provides a heating element which is small in volume and which can generate a large amount of heat radiation without overheating and breaking.

The invention thus provides a heater comprising the above described heating element and particularly useful in air heater, dryers inclusive of a hair dryer, air towel, liquid heater, humidifer, volatilizer and the like.

The principal object of the invention, therefore, is to provide a heating element comprising a PTC ceramic article composed of a honeycomb structural body.

Another object of the invention is to provide a heater, which comprises a heating element comprising a PTC ceramic article composed of a honeycomb structural body and particularly useful as an air heater, dryers inclusive of a hair dryer, air towel, liquid heater, humidifer, volatilizer and the like.

A further object of the invention is to provide various types of heaters each having a preferable construction.

A still further object of the invention is to provide a method of producing a heating element comprising a PTC ceramic article composed of a honeycomb structural body and to provide a method of forming ohmic electrodes therefor.

Other objects, advantages and capabilities of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings showing only a preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a preferred embodiment of the heating element according to the invention;

FIG. 2 is a section along line IIII in FIG. 1;

FIG. 3 is a perspective view of another embodiment of the heating element according to the invention;

FIG. 4 is a section along line IV-IV in FIG. 3;

FIG. 5 is a graph which illustrates the relation between the amount of heat radiated from the heating element according to the invention and the amount of air fed to the heating element;

FIG. 6 is a perspective view of a preferred embodiment of an air heater employing the heating element according to the invention, a part being broken away for clarity;

FIG. 7 is a perspective view of a preferred embodiment of a liquid heater employing the heating element according to the invention, a part being broken away for clarity; and

FIG. 8 is a perspective view of a preferred embodiment of a humidifier employing the heating element according to the invention, a part being broken away 3 for clarity.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiments of the invention will now be described in greater detail with reference to the accompanying drawings.

In FIGS. 1 and 2 a heating element 1 is shown according to the invention which is composed of a honeycomb structural body provided with a multiplicity of channels 2 extending therethrough and generally parallel to each other. Each of these channels 2 is bounded by a partition wall 3 which is substantially uniform in thickness. The honeycomb structural body constructed as above described insures extremely high surface-tovolume ratios. The partition walls 3 are provided at their opposed end surfaces substantially perpendicular to theaxial direction of the channels 2 with a pair of ohmic electrodes 4, 5, respectively. The thickness of both the partition wall 3 and ohmic electrodes 4, 5 is shown in enlarged scale in FIGS. 1 and 2 for clarity.

A preferred method of producing the heating element according to the invention will now be described.

BaCo TiO SiO and La O were weighed with mole ratios of l.00:1.02:0.02:0.003, respectively, mixed in a ball mill with rubber lining for 12 hours, dried and calcined at 1,100C for 3 hours. The calcined raw material was roughly pulverized by a double roll crusher with alumina rolls and then finely pulverized in a ball mill with rubber lining for 6 hours. The finely pulverized particles were passed through a screen with a mesh of 149 microns and then dried. To 100 parts by weight of the dried powders were added 4 parts by weight of methyl cellulose, 16.5 cc of 12% polyvinyl alcohol water solution, 3 parts by weight of polyethylene glycol and 8.5 parts by weight of water. The mixture was well blended in a kneader and then subjeted to de-ain'ng while blending with the aid of a de-airing pug mill. The product thus obtained was extruded from the de-airing pug mill through a honeycomb structural body forming nozzle so as to form a green body composed of a honeycomb structural body. The green body thus obtained was dried by means of a freeze drying method using dry ice and then fired in a standard electric furnace using silicon carbide heaters at l,350 C for 2 hours. A column shaped honeycomb structural body having a diameter of 4 cm and a thickness of 1 cm (length of channels) and provided with a multiplicity of channels each square in section having each side of 0.195 cm and bounded by a partition wall having a thickness of 0.03 cm was thereby obtained. The honeycomb structural body has surface-to-volume ratios of about 16 cm /cm and total surface area of about 200 cm Ohmic electrodes were provided for each end surface substantially perpendicular to the axial direction of the channel, thus completing a heating element.

The, ceramic compositions are not restricted to the above described ceramic composition and any other customary compositions which can obtain a positive temperature coefficient of electric resistance may also be used. Examples of such ceramic compositions are those described in U.S. Pat. Application Ser. No. 431,397 filed on Jan. 7, 1974 which is a continuationin-part of Application Ser. No. 256,368 filed on May 24, 1972; now abandoned and those described in U.S. Pat. No. 2,981,699, No. 3,373,120 and No. 3,441,517, respectively.

In addition, the honeycomb structural body may be formed by any methods other than the above described extrusion method. Examples of these methods are a press method, a method of producing a bundle of a number of green ceramic pipes and then firing the bundle, or a method of forming a honeycomb structural body comprising coating ceramic raw material particles suspended in an organic binder on the surface of sheet of paper and the like, corrugating the coated sheet of paper and the like, accumulating the corrugated sheet of paper and the like and firing the accumulated sheet of paper and the like as disclosedin U.S. Pat. No. 3,112,184.

In addition, the sectional configuration of the channels may be formed into any configurations other than square as shown in FIGS. 1 and 3, such, for example, as triangular, hexagonal or any other polygonal, circular and the like sectional configurations (not shown).

The outer configuration of the honeycomb structural body constituting the essential part of the heating element is shown as columnar in shape in FIG. 1 and rectangular parallelepipe in shape in FIG. 3, but if necessary the outer configuration of the honeycomb structural body may be of cube, polygonal columnar in shape (not shown).

In the embodiment shown in FIGS. 1 and 2, the honeycomb structural body is provided at its each end surface substantially perpendicular to the axial direction of the channels with the ohmic electrode. But, these ohmic electrodes may be mounted on any positions other than end surfaces of the honeycomb structural body.

For example, the honeycomb structural body may be provided at its each side surface 6, 7 which is substantially parallel to the axial direction of the channels and opposed to each other with a pair of electrodes 8, 9 as shown in FIGS. 3 and 4. Whether the ohmic electrodes should be mounted on the surfaces substantially perpendicular to the axial direction of the channels or mounted on the outer surfaces of the honeycomb structural body substantially parallel to the axial direction of the channels may be determined depending on the outer configuration of the honeycomb structural body its mechanical strength, the purpose to which the honeycomb structural body is used, the amount of heat radiation, the restriction in design of the heater into which the honeycomb structural body is incorporated as a heating element and any other various conditions.

In FIGS. 2 and 4, the thickness of the partition wall 3 and ohmic electrodes 8, 9 is shown in enlarged scale for clarity.

The ohmic electrodes may be formed on the outer surfaces of honeycomb structural body by means of such a common method as those described, for example, in U.S. Pat. No. 3,676,211, British vPat. No. 1,252,490, Electrodes for Ceramic Barium Titanate Type Semiconductors by H. M. Landis, Journal of Applied Physics, 1965, Vol. 36, Nos. 6, pages 2,000 to 2,001 and the like. But, it is preferable to form the ohmic electrodes on the outer surfaces of the honeycomb structural body by means of the following method in order to produce the heating element according to the invention.

' When forming the ohmic electrodes on the surfaces substantially perpendicular to the axial direction of the channels as shown in FIGS. 1 and 2, it is preferable to use the silver paste baking method, the aluminum hot spraying method or electroless nickel plating method.

When forming the ohmic electrodes on the outer surfaces of the honeycomb structural body substantially parallel to the axial direction of the channels as shown in FIGS. 3 and 4, it is preferable to use the aluminum hot spraying method. These methods will now be described, respectively.

The silver paste baking method comprises coating silver paste on the ceramic article by means of a screen printing method and-baking the coated surfaces. However, silver paste having the typical composition used with a customary condenser, for example, Silver Paste 7095 made by E. I. Du Pont de Nemours and Company, could not provide the ohmic electrodes which are well suited for the PTC ceramic article. Such a disadvantage can be obviated by the use of a silver paste containing indium. On the other hand, such indium containing silver paste is expensive.

The silver paste developed by the invention consists of silver and zinc with a range of weight ratios of 2:1 to 40:1, glass powder, and an organic solvent. This silver paste after coating is baked at a temperature of 420 to 550C.

It is preferable to apply such silver paste baking method to a PTC ceramic article whose Curie Temperature (Tc) is lower than a temperature of about 150C. This is because of the fact that silver is subjected to surface diffusion at a temperature higher than 200C so that there is a risk of the silver coated surfaces being shortcircuited to each other at a temperature higher than 200C after long use.

The aluminum hot spraying method is capable of providing electrodes which can eliminate the above described difficult problem which has been encountered with the silver paste baking method. In the aluminum hot spraying method, it has been the common practice to spray aluminum against surfaces of a ceramic article in a direction substantially perpendicular thereto so as to expedite adherence of aluminum to those surfaces, as described in the US. Pat. No. 3,676,211. If aluminum is sprayed from a direction perpendicular to the surfaces which is substantially perpendicular to the axial direction of the channels of the honeycomb structural body, that is, from the direction substantially parallel to the axial direction of the channels, aluminum becomes adhered 'to the inner wall surfaces of all of the channels, thereby shortcircuiting the sprayed surfaces. In order to obviate such disadvantage, the invention provides an improved method wherein aluminum is sprayed in a direction inclined from the surfaces which is substantially perpendicular to the axial direction of the channels by an angle of to 60, preferably, to 45.

In the electroless nickel plating method, the ceramic article is plated with nickel as a whole. Such method, therefore, could not be applied to the invention. Thus, the invention provides an improved electroless nickel plating method by which nickel is plated on surfaces only substantially perpendicular to the axial direction of the channels and which comprises immersing the honeycomb structural body into silicon resin or wax, for example,,to mask the total body with the resin or wax, grinding the surfaces substantially perpendicular to the axial direction of the channels to remove the mask, subjecting activating treatment only to the mask removed surface, and immersing the body into nickel salts solution to form ohmic electrodes on the surfaces only substantially perpendicular to the axial direction of the channels. In this method, it is important to mask the honeycomb structural body with the resin or wax such that each channel is maderound in shape at its corner edges. These masked round corner edge portions can prevent a penetration of the activating treating liquid into the channels due to surface tension and hence prevent an electroless plating against the inner walls of the channels, thereby preventing shortcircuit across the electrodes.

A heater employing the heating element according to the invention will now be described in greater detail.

As an example, use is made of a heating element comprising a PTC ceramic article composed of a honeycomb structural body which is provided with a multiplicity of channels each square in cross section with a side of 0.125 cm and bounded by partition walls whose thickness is 0.02 cm. The particulars of the PTC ceramic article are as follows. The diameter is 4 cm, thickness (length of each channel) 1 cm, Tc=lC, the surface-to-volume ratios about 24 cm /cmi and the total surface area about 300 cm The heating element is provided at its surface substantially perpendicular to the axial direction of each channel with ohmic electrodes.

In FIG. 5, a curve 101 graphically illustrates the relation between the amount of air Q (m /min) fed into the channels of the heating element at 20C and the amount of heat P (watt) radiated from the above mentioned heating element. As seen from the curve 101, the amount of heat radiation is about 230 watts when the amount of air fed is 0.1 m /min, whereas if the amount of air fed is increased to 0.5 m /min which is 5 times larger than 0.1 m /min, the amount of heat radiation becomes also increased to about 450 watts. If the amount of air fed is substantially zero, that is, if the forced draft is stopped and use is made of natural convection air caused by the heat radiated from the heating element, the amount of heat radiation becomes only 20 watts.

As described above, when various kinds of fluid inclusive of air pass'through a multiplicity of channels of the heating element according to invention, the heating element with a small volume can radiate a large amount of heat. Thus, a heater comprising the heating element according to the invention and means for feeding fluid through the heating element such, for example, as'a fan, pump, water pressure of city water and the like can prevent an overheating of the heating element when the fluid feeding means becomes stopped without intentionally providing a safety device such as a tempera-' ture fuse, thermostat and the like.

In addition, in the heater of this invention (when use in combination with means for feeding fluid through the heating element) is capable of changing the amount of heat radiation by varying the amount of fluid. It is possible, therefore to control a large electric power with the aid of a small electric power.

For example, if use is made of air as fluid, the amount of air on the order of 0.2 m /min can be fed by means of a fan driven by an electric motor of about 20 watts. As a result, as seen from FIG. 5, the control of the 20 Watts motor in its rotating speed ensures a control of the amount of heat radiation corresponding to electric power of 300 Watts which is about 15 times larger than 20 Watts.

In the heating element according to the'invention, it is important to make the surface-to-volume ratios'of the honeycomb structural body large. Even when the amount of passing air per unit volume of the heating element is the same, the larger the surface-to-volume ratio the larger the amount of heat radiation. For comparison, use is made of another heating element comprising a PTC ceramic article composed of a honeycomb structural body which is provided with a multiplicity of channels each square in cross section with a side of 0.195 cm and bounded by partition walls whose thickness is 0.03 cm. The particulars of the PTC ceramic article are as follows. The diameter is 4 cm, thickness (length of each channel) 1 cm, Tc--190C, the-surface-to-volume ratios about 16 cm /cm and the total surface area about 200 cm This element is made of the same ceramic composition as those of the body from which the curve 101 was derived. The heating element is provided at its surface, substantially perpendicular to the axial direction of each channel, with fohmic electrodes. In FIG. 5, a curve 102 graphically illustrates the relation between the amount of air Q (m /min) fed into the channels of the above mentioned heating element at 20C and the amount of heat P (Watt) radiated from the heating element. A comparison between the curves 101 and 102 clearly shows that the amount of heat radiation P (watt) shown by the curve 101 which is plotted when the surface-to-volume ratios are about 24 cm /cm is larger than that shown by the curve 102, which is plotted in the case of the surface-to-volume ratios are about 16 cm /cm The frictional resistance of the fluid passing through a multiplicity of channels against the honeycomb structural body is proportional to the surface-to-volume ratios under the same configuration of the channels, so that the'surface-to-volume ratios should be determined in association with the kinds of fluid and means for feeding fluid through the heating element. It is preferable to determine the surface-to-volume ratios to a range from to 60 cm /cm with respect to the honeycomb structural body of the heating element according to the invention. Any structural body having some holes made, for example, by boring could neither make the thickness of the partition will bounding the holes substantially uniform, nor obtain the surface-to-volume ratios in the range of 10 to 60 cm /cm The Curie Temperature (Tc) of the PTC ceramic article of the heating element according to the invention is determined according to what purposes the heating element is used. If the heating element is used for a handy hair dryer, the heating element is restricted in its size. As a result, the Curie Temperature (Tc) of the PTC ceramic article is made relatively high that a heating element small in volume can radiate a desired amount of heat. The amount of heat radiation is associated not only with the Curie Temperature (Tc) of the PTC ceramic article, but also with the surface-tovolume ratios of the honeycomb structural body. It is preferable to make the Curie Temperature (Tc) 150 to 200C for the handy hair dryer.

As will be described later, if the heating element according to the invention is used for a heater, a plurality of heating elements may be used according to the desired amount of heat radiation and to the other designs. In this case, a customary heating element made of nickel-chrome wire, for example, may be included in a plurality of heating elements.

.Though these heating elements may electrically connected in series and/or in parallel with each other by techniques known in the art, it is-found to be preferable to connect in parallel from our investigation.

In FIG. 6 is shown a preferred embodiment of the heating element according to the invention as applied to an air heater.

Referring to FIG. 6, an air heater 10 comprising a heating element 1, and means for feeding fluid through the heating element 1 such as a fan 12 driven by a motor 11. Such means can feed air into a multiplicity of channels of the heating element 1. Means for feeding fluid through the heating element 1 is settled to be aligned with the channels of the honeycomb structural body of the heating element 1 and the assembly is enclosed in a housing 13. In the present embodiment, arrangement and electrical connections are so designed that the heating element 1 is provided at its opposed surfaces substantially perpendicular to the axial direction of the channels with ohmic electrodes, and that the heating element 1 is sandwiched between a pair of terminal plates 14 and 15. It is a matter of course that insulating spacers (not shown) are inserted-between these terminal plates 14, 15 and the housing 13 so as to electrically insulate the former from the latter.

The air heater 10 shown in FIG. 6 will operate as follows. If a switch 16 is turned ON, the fan 12 is rotated to suck air through an inlet opening 17 adapted to control the amount of air passing therethrough into the heating element 1. At the same time, electric current is supplied to the heating element 1 to bring it into a heat radiating condition, and as a result, the air passing through the channels is heated and blown out of the air heater 10.

Between ON and OFF, a state which the fan 12 continues its air feeding operation and the heating element 1 stops its heating operation may be employed.

During heating of the heating element 1 by the current supplied thereto, even if the fan 12 becomes stopped or the air inlet opening 17 gets cloggedby a towel, a curtain or the like, to cause no air to pass through the multiplicity of channels of the heating element 1, the amount of heat radiated from the heating element 1 becomes small in the manner as described above. As a result, there is no risk of the heating element 1 being overheated or of a fire being caused without intentionally providing a safety device such as a temperature fuse, thermostat and the like. The air heater shown in FIG. 6 with or without any modification may be used as a hair dryer, domestic or industrial dryers, air towel, room heater and the like.

In FIG. 7 is shown another preferred embodiment of the heating element according to the invention as aparea of about 1,500 cm and ohmic electrodes pro-' vided on the side surfaces of the honeycomb structural body which is substantially parallel to the axial direction of each channel, and heat insulating material 21 covering the heating element 1 and enclosed in a housing 22. In this embodiment, a pipe 20 is also provided for feeding water to the channels. The inlet pipe 19 is connected to, for example, a city water faucet (not shown) to feed water into-the pipe 20, so that the water passing through the multiplicity of channels is heated and flows out of an outlet pipe 23.

. The liquid heater 18 shown inFlG. 7 will-operate as follows..If the switch 16 is turned ON, heat is radiated from the heating element 1 and hot water flows out of the outlet pipe 23. f a

If the liquid heater 18 was fed with city water at 20C at a rate of about-l l/min and at the same time the pair of electrodes were applied "with voltage-of 240 volts, the city water heated to about 60C flowed from the outlet pipe 23. In this case, the amount of heat radiation was 2.9 kilowatts. The amount of heat radiated when the water supply was stopped became smaller than 100 watts, thereby involving no danger due to overheating of the heating element 1.

The electrical insulation of both the electrodes against an electric conductive liquid such as water may be effected by coating the heating element 1 as a whole, exclusive of the lead wires led out of the electrodes, with corrosion resistant materials such as fluorine resin (Teflon made by E. I. Du Pont de Nemours and Company, Polyflon, Daiflon and Neoflon made by Daikin Kogyo Co., Ltd. Osaka, Japan and the like), fluorine rubber (Viton made by Du Pont), silicone resin, silicone rubber, silicone varnish and the like.

In addition, such coating can protect the heating element 1 from being corroded by corrosive liquid such as acids, alkalis and the like.

Alternatively, as means for preventing the heating element 1 from being corroded by the corrosive fluid, provision may be made for a pipe having a small diameter and made of anti-corrosion material such as stainless steel and this pipe may be inserted into each of the channels of the heating element whereby the corrosive fluid can pass through the pipe without making contact with the inner surface of each of the channels.

The liquid heater shown in FIG. 7 is not provided with means for feeding fluid through the heating element, such as provided for the air heater shown in FIG. 6, but the city water pressure or gravity serves as means for feeding liquid through the heating element. In addition, the liquid heater shown in FIG. 7 is connected to the city water faucet and adapted to heat the city water. But, the liquid heater shown in FIG. 7 may also be used as a heater for drinks such as Japanese Sake and the like, and an oil preheater for heating liquid fuel and the like.

In FIG. 8 a further preferred embodiment of the heating element according to the invention is shown as applied to a humidifier.

Referring to FIG. 8, a humidifier 24 comprising a housing 26 enclosing water contained therein and a heating element 1. In the present embodiment, it is important that one of the surfaces substantially perpendicular to the axial direction of the channels of the honeycomb structural body is always in contact with the water 25. In order to make one of the surfaces substantially perpendicular to the axial direction of the channels of the honeycomb structural body always contact with the water 25, provision is made for a water absorbing mat 27 which is supported in a space above the water 25 and on which is disposed the heating element 1. The water absorbing mat 27 is provided at its periphery with a bundle of fibers 28, such as woven cloth secured at its one end to the mat 27, the other end being immersed into the water 27. The fiber bundle 28 and mat 27 permit the water to be supplied to the surface substantiallyperpendicular to the axial direction of the channels of the honeycomb structuralbody with the aid of capillary phenomenon of the fiber bundle 28 and the mat 27;

The humidifier shown in FIG. 8 'will operate as follows; If the switch 16 is turned ON, the water in contact with one of the surfaces substantially perpendicular to the axial directionof the channels of the honeycomb structural body of the heating element 1 is heated by the heating element 1 and converted into steam which is exhausted from an opening 29 provided at the'top of the housing 26 and arranged above the heating element 1.

In the present embodiment, use was made of a heating element comprising a PTC ceramic article composed of a column shaped honeycomb structural body provided with a multiplicity of channels each square in cross section with a side of 0.125 cm and bounded by partition walls whose thickness is 0.02 cm, the honeycomb structural body having a diameter of 3.5 cm, a thickness (length of each channel) of 1 cm, Curie Temperature (Tc) of 190C, surface-to-volume ratios of about 24 cm /cm and total surface area of about 240 cm and ohmic electrodes provided on both surfaces substantially perpendicular to the axial direction of each channel. Water at 20C was treated by applying voltage of volts across the ohmic electrodes. The water was evaporated at a rate of 4 cc/min. The housing 26 is provided at its side wall with a window 30 through which can peep the change in liquid level and provided at its cover with an opening 31 through which is added liquid when the liquid level becomes lower than a desired level.

The humidifier shown in FIG. 8 with or without modifications may be used as a volatilizer, distillator, fractionator and the like for use with domestic or industrial water and any other liquids such as oil.

While several examples have been herein disclosed, it is obvious that various changes can be made without departing from the spirit and scope of the invention as set forth in the appended claims. Further, it is to be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A heating element consisting essentially of:

a. a column-shaped honeycomb structural body of electrically conductive ceramic material, said body having a multiplicity of substantially uniform parallel channels extending therethrough with each of said channels being bounded by a partition wall which is substantially uniform in thickness, said structural body also having a surface-to-volume ratio in the range of 10 to 60 cm /cm and having a positive temperature coefficient of electrical resistance;

b. a pair of ohmic electrodes mounted on the opposite surfaces of the body and in electrical contact therewith;

0. Means for feeding fluid through said channels.

2. The heating element of claim 1 in which the ohmic electrodes are mounted on opposite surfaces substantially perpendicular to the axial direction of the channels.

3. The heating element of claim I in which the surface to volume ratios are 10 to 40 cm /cm 4. The heating element of claim 3 in which the surface to volume ratios are 12 to 30 cm /cm 5. A fluid heater which comprises:

b. A pair of ohmic electrodes mounted on the opposite surface of the body;

c. Means for feeding fluid through said channels.

6. The heater of claim 5 in which the ohmic electrodes are mounted on the opposite end surfaces substantially parallel to the axial direction of the channels.

7. The heater of claim 5 in which the surface to volume ratios are 10 to 40 cm /cm 8. The heater of claim 7 in which the surface to volume ratios are 12 to 30 cm /cm UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 3 DATED DGC'B'ZiiDGl" 1.5, 1975 INVENTORKS) I Nada et a1 It is certified that error appears in the above-identified potent and that said Letters Patent are hereby corrected as shown below:

Delete column 1 line 5 sai fi channels.]

r, do} I-ieans for feeding fluid through Signed and Sealed this Arrest:

RUTH C. MASON C. MARSHALL DANN Amnrrrrrg Officer (mnmissimu'r nflau'nrr umi Trarlvmurkx UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT 3, 2 i73- DATED DQ681108? 1.5:, 1f??? INVENTORKS) i r ggt a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below Delete column 1-4,), line 59, {(CF t'leans fer fluid through said channelsj Signed and Scaled this eleventh Of May1976 [SEAL] Arrest.

RUTH c. MASON c. MARSHALL DANN Arrr'srmg ()jji'cer (ummr'xsimu'r ujlarenrs and Trademarks 

1. A heating element consisting essentially of: a. a column-shaped honeycomb structural body of electrically conductive ceramic material, said body having a multiplicity of substantially uniform parallel channels extending therethrough with each of said channels being bounded by a partition wall which is substantially uniform in thickness, said structural body also having a surface-to-volume ratio in the range of 10 to 60 cm2/cm3 and having a positive temperature coefficient of electrical resistance; b. a pair of ohmic electrodes mounted on the opposite surfaces of the body and in electrical contact therewith; c. Means for feeding fluid through said channels.
 2. The heating element of claim 1 in which the ohmic electrodes are mounted on opposite surfaces substantially perpendicular to the axial direction of the channels.
 3. The heating element of claim 1 in which the surface to volume ratios are 10 to 40 cm2/cm3.
 4. The heating element of claim 3 in which the surface to volume ratios are 12 to 30 cm2/cm3.
 5. A fluid heater which comprises: a. a column-shaped body of electrically conductive ceramic material said ceramic body having a multiplicity of substantially uniforms parallel channels extending therethrough with each of said channels being bounded by a partition wall which is substantially uniform in thickness, said body having a surface to volume ratio in the range of 10 to 60 cm2/cm3, said ceramic material having a positive temperature coefficient of electrical resistance; b. A pair of ohmic electrodes mounted on the opposite surface of the body; c. Means for feeding fluid through said channels.
 6. The heater of claim 5 in which the ohmic electrodes are mounted on the opposite end surfaces substantially parallel to the axial direction of the channels.
 7. The heater of claim 5 in which the surface to volume ratios are 10 to 40 cm2/cm3.
 8. The heater of claim 7 in which the surface to volume ratios are 12 to 30 cm2/cm3. 